This invention relates to electronic instruments for detecting and measuring RF voltage wave signals on coaxial transmission lines such as between a transmitting antenna and a transmitter. More particularly, the invention relates to an "insertion-type" RF directional wattmeter for detecting and measuring both the forward and reflected voltage wave signals on a coaxial transmission line.
Insertion-type RF directional wattmeters are used in many applications in the RF field, particularly, in matching antennas to transmission lines and in minimizing the voltage standing wave ratio (VSWR) on the line. One application that is becoming increasingly important is in connection with CB transmitters that are currently so popular in the United States for use in automotive vehicles. These installations require accurate impedance matching in view of the limited power permitted.
Meters currently available for this application are, for example, of the type disclosed in U.S. Pat. Nos. 2,852,741 and 2,891,221.
In these units a rigid, coaxial line section is inserted in the coaxial transmission line such as by standard coaxial connectors and an inductive pick-up coil positioned in a transverse opening in the outer conductor of the line section is adapted for rotation about an axis normal to the axis of the line section. The pick-up coil is connected by special leads to a D'Arsonval meter movement and the resulting meter reading indicates the magnitude of the wave signal in watts, the indication being either that of the magnitude of the forward voltage wave level or the reflected voltage wave level depending upon the particular orientation of the pick-up coil.
The pick-up coil is located in the electrical field between the inner and outer conductors of a coaxial transmission line and has a voltage induced therein proportional to the current I in the inner conductor, there being a mutual inductance M between the loop and the transmission line and the loop being positioned in the plane of the inner conductor of the line. A series circuit of resistance R and capacitance C connected across the transmission line conductors will give a voltage across the resistance R proportional to the voltage E between the line conductors. In directional couplers and so-called reflectometers the arrangements mentioned are combined in a sampling circuit in which the resistor R is connected in series with the loop and capacitive coupling is provided as by capacitor plates or armatures on the loop and the inner conductor or by capacitance effects between the components of the sampling circuit and the inner conductor.
Considering the sampling circuit mentioned and using lumped impedances, it is apparent that M is either positive or negative depending upon the directional relation between the loop and the wave signal energy traveling on the line.
The instrument described obtains reversal of the mutual inductance M through 180.degree. rotation of the loop relative to the transmission line. The forward traveling wave has voltage E.sub.f and current I.sub.f while the reflected traveling wave has voltage E.sub.r and current I.sub.r. Thus, if Z.sub.o be the characteristic impedance of the line and p the reflection coefficient: EQU p = E.sub.r /E.sub.f = -I.sub.r /I.sub.f
and EQU e = jw (CRE + MI) EQU = jwE.sub.f [CR (1+p) + (M/Z.sub. o) (1-p)]
where e is the total electromotive force induced in the loop or sampling circuit. The components are selected so that: EQU RC = M/Z.sub.o = K
k being a constant. If we let e be the electromotive force when M is positive so that the voltage across R and the voltage induced in the loop are additive, and let e- be the electromotive force when M is negative and the voltages referred to are opposed, the former gives a maximum and the latter a minimum indication, thus: EQU e = jwE.sub.f [K(1+p) + K(1-p)] EQU = 2jwE.sub.f K EQU e- = jwE.sub.f [K(1+p) - K(1-p)] EQU = 2jwE.sub.f K.sub.p
from which the reflection coefficient and standing wave ratio can be obtained. It is also feasible to measure power P being fed through the transmission line: ##EQU1##
When it is desirable to build a power meter with variable sensitivity in order to measure ratio or VSWR directly, the usual method is to incorporate a control into the meter circuit between a rectifier diode and the D'Arsonval meter movement.
When it is desirable to build a power meter with variable sensitivity in order to measure ratio or VSWR directly, the usual method is to incorporate a control into the meter circuit between a rectifier diode and the D'Arsonval meter movement.
Since the output of the coupler is proportional to .sqroot.power the output of the diode is given by K.sub.1 .sqroot.power - f(diode) where f(diode) is not proportional to power. The control varies the sensitivity of the metering circuit so that meter deflection is given by: ##EQU2##
K.sub.2 cannot be factored out and brought to the left hand side of the equation, so the control (K.sub.2) does not multiply the power of all points on the scale by the same constant.
The instrument of the present invention avoids the difficulties indicated above and affords other features and advantages heretofore not obtainable.